The present invention is an improvement of what has been described in European Patent Application No. 01830645.6, which is incorporated herein by reference and is assigned to the current assignee of the present invention. The growing interest shown by the international community for the decrease of air pollutants has led to the issuing of more and more severe regulations concerning automobile vehicle polluting emissions.
In particular, the European Union plans to implement within 2005 severe restrictions on exhaust emissions and fuel consumption of internal combustion engines. The most significant regulations are briefly described below, and some of them are already in force while others are pending:
Euro III (98/69): vehicles registered from Jan. 1, 2001 comply with this directive. Besides the problem of pollutant emission, which is lower than the previous ones, this directive introduces the requirement of an on board autodiagnostic system OBD (On Board Diagnostic), indicating any malfunction. It is compulsory to do the repair within a determined number of kilometers, otherwise harsh sanctions are applied. These directives, which are valid for petrol cars, will come into force in 2003 for diesel engines.
Euro IV (98/68 B): it will come into force on Jan. 1, 2005. Euro V (2001/27/EC): it will come into force on Jan. 1, 2008.
Vehicle emissions highly depend on the rotational speed due to the engine use, such as driving in the city, in the country or on a freeway, for example. In the future, compliance with these regulations will involve a considerable effort by car producers in developing low emission vehicles. In this point of view, hybrid propulsion vehicles will play a leading role in consideration of both the more developed technology and the low emissions, but also of the lower consumption.
The prior art already provides some configurations of hybrid propulsion vehicles, i.e., vehicles equipped with an electric engine and an internal combustion engine. The two conventional hybrid vehicle configurations are the series configuration and the parallel configuration.
In the series configuration the internal combustion engine runs at a peak efficiency steady state to recharge the storage batteries powering the electric engine. Essentially, the engine operates as a generator and it is sized according to the drive-demanded average power.
It is evident that this power value is considerably lower than the highest deliverable power. Therefore, under such conditions, the internal combustion engine operates at a torque curve point having the highest efficiency and wherein polluting emissions are reduced to a minimum.
In this configuration, the electric machine mounted in a vehicle runs mainly as an engine, and runs as a generator only during the regenerative braking steps. The electric machine rating must be equal to the vehicle rating, since the drive demanded power is supplied only by the electric engine.
The drawbacks of this configuration are represented by the batteries which, having to be sized according to the electric machine rating, will be characterized by considerable size and weight, negatively affecting the vehicle performances. FIG. 1 shows in schematic blocks the structure of a hybrid propulsion vehicle of the previously described series type.
In the parallel configuration the internal combustion engine runs dynamically (not at a fixed point) and it contributes, together with the electric drive, to supply the required mechanical power. Generally, the internal combustion and electric engine contributions are delivered to the wheel axis through a torque conversion mechanical coupling.
The total vehicle power is thus distributed between the electric engine and the internal combustion engine. Therefore, the latter power is lower than the one of a conventional vehicle engine, in consideration also of the possible electric machine overload.
The efficiency and the polluting emissions are optimized through an adequate control of the radiant flux distribution among the main components. The electric engine has a limited power and it operates also as a generator to recharge the batteries. The batteries have a reduced size and weight since they power a reduced power electric engine. FIG. 2 shows in schematic blocks the structure of a parallel-type hybrid propulsion vehicle.
Both of the above described series/parallel configurations have advantages and disadvantages. In the series configuration hybrid system the internal combustion engine only functions for the battery charge, therefore the high energy density of fossil fuels cannot be exploited. Moreover, the high weight of the storage batteries causes a considerable increase in the vehicle inertia and this damages the equal power performances.
Also, the need to use two different electric machines, the one for the drive and the other for the storage battery recharge, increases the system complexity to the detriment of reliability. On the contrary, the parallel configuration hybrid system requires controllers or mechanical elements allowing the overall torque required to be distributed between the thermal engine and the electric engine.
The torque distribution methods according to the prior art do not ensure a global optimization of power output but only an optimization linked to contingent situations. To obtain a global optimization, not only the path that will be covered by the vehicle but also all the driving conditions that will appear should be previously known in principle.
Another possible approach should be to make forecasts based on the past system history with probabilistic behavior evaluations. Nevertheless, this approach is difficult to implement and it would not lead to particularly interesting results from an industrial point of view.